Arthropod

NOTE: Some classification schemes group Myriapoda and Hexapoda into one subphylum called Uniramia.

Basic arthropod structure

The success of the arthropods is related to their hard exoskeleton, segmentation, and jointed appendages. The appendages are used for feeding, sensory reception, defense, and locomotion.

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Most arthropods respire (breathe) through a tracheal system (an exception being Pauropoda); a potential difficulty considering that the skeletal structure is external and covers nearly all of the body. Aquatic arthropods use gills to exchange gases. These gills are specialized with an extensive surface area in contact with the surrounding water. Terrestrial arthropods have internal surfaces that are specialized for gas exchange. The insects have tracheal systems: air sacs leading into the body from pores, called spiracles, in the cuticle.

Related Topics:
Tracheal - Pauropoda - Aquatic - Gills - Terrestrial - Pore - Spiracles - Cuticle

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Arthropods have an open circulatory system. Hemolymph, a copper-based blood analogue, is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues. Arthropods are protostomes. There is a coelom, but it is reduced to a tiny cavity around the reproductive and excretory organs, and the dominant body cavity is a hemocoel, filled with hemolymph which bathes the organs directly. The arthropod body is divided into a series of distinct segments, plus a presegmental acron which usually supports compound and simple eyes and a postsegmental telson. These are grouped into distinct, specialized body regions called tagmata. Each segment at least primitively supports a pair of appendages.

Related Topics:
Open circulatory system - Hemolymph - Copper - Blood - Protostome - Coelom - Compound

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The cuticle in arthropods forms a rigid exoskeleton, composed mainly of chitin, which is periodically shed as the animal grows. They contain a inner zone (procuticle) which is made of protein and chitin (a polysaccharide) and is responsible for the strength of the exoskeleton. The outer zone (epicuticle) lies on the surface of the procuticle. It is nonchitinous and is a complex of proteins and lipids. It provides the moisture proofing and protection to the procuticle. The exoskeleton takes the form of plates called sclerites on the segments, plus rings on the appendages that divide them into segments separated by joints. This is in fact what gives arthropods their name—joint feet—and separates them from their very close relatives, the Onychophora and Tardigrada. The skeletons of arthropods strengthen them against attack by predators and are impermeable to water. In order to grow, an arthropod must shed its old exoskeleton and secrete a new one. This process, molting, is expensive in energy consumption. During the molting period, an arthropod is vulnerable. Once their cuticle hardens they are fully developed and can never grow again. Their cuticles slowly expand as they increase in mass. They breakdown (digest) their cuticle every now and then when they need to grow. Their cuticle hardens at their adult size and they slowly grow to fill it up.

Related Topics:
Chitin - Proteins - Lipids - Onychophora - Tardigrada

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